1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems, and more particularly to mechanisms and techniques, for performing a marine seismic survey using underwater nodes that carry appropriate seismic sensors.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration for determining the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional marine system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 located on streamers 12. The streamers may be disposed along any depth profile relative to the surface 14 of the ocean. The streamers may have spatial arrangements other than horizontal. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward and penetrates the seafloor 20 until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on further analyses of the data collected by the receiver 11, an image of the subsurface is generated. The seismic source array 16 includes plural individual source elements.
FIG. 2 shows a vessel 40 towing two cables 42 at respective ends with deflectors 44. Plural lead-in cables 46 are connected to streamers 50. The plural lead-in cables 46 are also connect to the vessel 40. The streamers 50 are maintained at desired distances from each other by separation ropes 48. Plural individual source elements 52 are also connected to the vessel 40 and to the lead-in cables 46 via ropes 54.
However, this traditional configuration is expensive because the cost of streamers is high. In addition, this configuration might not provide accurate results because water surface noise may interfere with recordings. To overcome these problems, new technologies deploy plural seismic sensors on the bottom of the ocean to create a coupling between the sensors and the ocean floor.
One such technology is incorporated into the ocean bottom station (OBS). An OBS is capable of providing better data than conventional acquisition systems because of its wide-azimuth geometry. Wide-azimuth coverage is helpful for imaging beneath complex overburdens like those associated with salt bodies. Salt bodies act like huge lenses, distorting seismic waves that propagate through them. To image subsalt targets, it is preferable to have the capability to image through complex overburdens, but even the best imaging technology alone is not enough. Good illumination of the targets is necessary. Conventional streamer surveys are operated with a single seismic vessel and have narrow azimuthal coverage. If either the source or the receiver is located above an overburden anomaly, some targets are likely to be poorly illuminated. OBS nodes can achieve wide-azimuth geometry.
Additionally, OBS nodes are much more practical in the presence of obstacles such as production facilities. For the purpose of seismic monitoring with repeat surveys (4D), OBS nodes have better positioning repeatability than streamers. Furthermore, OBS nodes provide multi-component data. Such data can be used for separating up- and down-going waves at the seabed, which is useful for multiple attenuations and for imaging using the numerous pieces of data. In addition, multi-component data allows for the recording of shear waves, which provide additional information about lithology and fractures and sometimes allow imaging of targets that have low reflectivity or are under gas clouds.
U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference, discloses this kind of node. In this case, the seismic sensors 60 are attached to a heavy pedestal 62, as shown in FIG. 3 (which corresponds to FIG. 4 of the patent). A station 64 that includes the sensors 60 is launched from a vessel and arrives, due to its gravity, on the ocean bottom. The station 64 remains permanently on the ocean bottom. Data recorded by the sensors 60 is transferred through a cable 66 to a mobile station 68. When necessary, the mobile station 68 may be brought to the surface to retrieve the data.
Although this method achieves better coupling between the seabed and the sensors, the method is still expensive and inflexible because the stations and corresponding sensors are left on the seabed. Also, the landing point of the station 64 cannot be controlled.
An improvement to this method is described in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, a sensor 70 (see FIG. 4) and a memory device 74 are removably attached to a pedestal 72. After recording the seismic waves, the sensor 70 and memory device 74 are instructed by a vessel 76 to detach from the pedestal 72 and rise to the ocean surface 78 to be picked up by the vessel 76.
However, this configuration is not very reliable, because the mechanism maintaining the sensor 70 connected to the pedestal 72 may fail to release the sensor 70. In addition, the sensor 70 and pedestal 72 may not reach their intended positions on the bottom of the ocean. Furthermore, the pedestals 72 are left behind, thereby contributing to both ocean pollution and a seismic survey price increase, which are both undesirable effects.
Accordingly, it would be desirable to have systems and methods that use inexpensive and non-polluting nodes for reaching a desired point on the seabed and recording seismic waves.